U.S. patent number 4,304,713 [Application Number 06/125,859] was granted by the patent office on 1981-12-08 for process for preparing a foamed perfluorocarbon dielectric coaxial cable.
This patent grant is currently assigned to Andrew Corporation. Invention is credited to Robert D. Perelman.
United States Patent |
4,304,713 |
Perelman |
December 8, 1981 |
Process for preparing a foamed perfluorocarbon dielectric coaxial
cable
Abstract
An improved foamable dielectric composition comprising a melt
extrudable perfluorocarbon resin containing therein (i) a
polytetrafluoroethylene (PTFE) nucleating agent and (ii) a volatile
blowing agent, preferably a lower fluorocarbon blowing agent having
only one or two carbon atoms; a process for forming such a
composition; and, products produced therewith. More specifically, a
foamed melt extrudable fluorinated ethylene-propylene (FEP) polymer
containing therein a PTFE nucleating agent and blown to a foamed
structure by means of a volatile fluid--e.g., a lower fluorocarbon
such as fluoromethane or, preferably, fluoroethane, a process for
forming such a foamed compostion; and, products produced therewith
such as jacketed electrical conductors and/or coaxial cables
wherein at least one conductor is bonded to such a foamed melt
extrudable resin. The foamed dielectric melt extrudable resin is
characterized by its thermal stability, fire resistance, small
closed cell structure, low density, and low insulating loss
characteristics. In a preferred embodiment, the melt extrudable
foamed resin comprises a perfluorocarbon polymer having a closed
cellular structure with cells ranging from 10 to 40 mils., a
density at least as low as 1.0 g./cc., and an insulation loss of
less than 1.8 db/100 ft. at 1000 MHz.
Inventors: |
Perelman; Robert D. (Hazel
Crest, IL) |
Assignee: |
Andrew Corporation (Orland
Park, IL)
|
Family
ID: |
22421768 |
Appl.
No.: |
06/125,859 |
Filed: |
February 29, 1980 |
Current U.S.
Class: |
264/45.9;
174/107; 174/110F; 174/110FC; 264/171.16; 264/53; 264/DIG.13;
264/DIG.5; 29/458; 29/828; 521/98 |
Current CPC
Class: |
C08J
9/0061 (20130101); C08J 9/14 (20130101); C08J
9/144 (20130101); H01B 13/142 (20130101); H01B
11/1839 (20130101); C08J 2327/12 (20130101); C08J
2427/00 (20130101); Y10S 264/13 (20130101); Y10T
29/49885 (20150115); Y10T 29/49123 (20150115); Y10S
264/05 (20130101) |
Current International
Class: |
C08J
9/00 (20060101); C08J 9/14 (20060101); H01B
11/18 (20060101); H01B 13/14 (20060101); H01B
13/06 (20060101); B29D 027/00 () |
Field of
Search: |
;264/53,DIG.13,45.9,174,DIG.5 ;521/98 ;29/458,828 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Slothour, "Expanded PTFE Dielectrics for Coaxial Cables", Plastics
Engineering, Mar. 1975, pp. 49-51. .
Sales lit. of Storm Products Co., entitled "Microporous Teflon
Coax". .
TR. No. 94 of E. I. duPont de Nemours & Co. (Inc.) Feb. 21,
1962 "Teflon 100 FEP-Fluorocarbon Resins, Method of Producing
Foamed Constructions". .
TR. No. 102 of E. I. duPont de Nemours and Co. (Inc.) "Method of
Extruding Foamed Shapes of `Teflon` 100, FEP-Fluorocarbon Resin"
(undated)..
|
Primary Examiner: Anderson; Philip
Attorney, Agent or Firm: Leydig, Voit, Osann, Mayer &
Holt, Ltd.
Claims
I claim as my invention:
1. A process for preparing the core for a foam dielectric coaxial
cable, said process comprising the steps of:
(a) forming a mixture of a melt extrudable fluorinated
ethylene-propylene polymer and a polytetrafluoroethylene nucleating
agent;
(b) heating the polymer and nucleating agent mixture to the molten
state;
(c) extruding the molten mixture through an extrusion die while
feeding a continuous conductor axially through the extrusion die so
as to form a layer of foam dielectric surrounding and in intimate
contact with the continuous conductor along the entire length and
about the entire periphery thereof;
(d) dissolving or injecting a volatile liquid blowing agent in the
melt extrudable polymer at a point in the process prior to exit of
the moisture from the extrusion die; and,
(e) recovering the foamed articles exiting from the extrusion
die.
2. The process of claim 1 wherein the fluorinated
ethylene-propylene polymer is a copolymer of tetrafluoroethylene
and hexafluoropropylene.
3. The process of claim 1 wherein the volatile liquid blowing agent
is a fluorocarbon.
4. The process of claim 1 wherein the fluorocarbon blowing agent is
a lower fluorocarbon having from 1 to 2 carbon atoms.
5. The process of claim 4 wherein the lower fluorocarbon is a
fluoroethane represented by the formula: ##STR5##
6. The process of claim 1 wherein the fluorocarbon blowing agent is
1,2-dichloro-1,1,2,2-tetrafluoroethane.
7. The process of claim 1 wherein on the order of 0.5 parts by
weight of polytetrafluoroethylene are contained within on the order
of 100 parts by weight of the fluorinated ethylene-propylene
polymer.
8. The process of claim 1 wherein the average particle size of the
polytetrafluoroethylene is less than 200 microns.
Description
BACKGROUND OF THE INVENTION
The present invention relates in general to an improved method for
forming products from a foamable dielectric composition, wherein
the composition comprises a melt extrudable perfluorocarbon resin
containing therein a polytetrafluoroethylene (PTFE) nucleating
agent for controlling the size of the cellular structure of the
foam, yet which does not significantly degrade the strength of the
resin and, consequently, which permits blowing the resin to a
density as low as 0.50 g./cc. with a suitable volatile fluid such,
for example, as a lower fluorocarbon preferably having one or two
carbon atoms--i.e., a fluoromethane represented by the formula:
##STR1## or, preferably, a fluoroethane represented by the formula:
##STR2## where X is selected from the group consisting of fluorine,
chlorine, bromine and hydrogen. The present invention is
particularly advantageous for use in the cable industry where, for
example, the inventive composition and process can be used to form
a foamed fluorinated ethylene-propylene (FEP) polymer--bonded to
one or more conductors; and, where the foamed polymer can comprise
either an insulating jacket surrounding one or more conductors
and/or a dielectric material used to hold the inner and outer
conductors of a coaxial cable in the desired spaced
relationship.
Coaxial cables with a foam dielectric between the inner and outer
conductors have been in commercial production since at least the
1950's. The dielectric loss of such cables has always been higher
than that of the so-called "air dielectric" cables which use solid
dielectric elements such as beads, helixes, or the like to hold the
inner and outer conductors in the desired spaced relation; but, the
disadvantage of higher dielectric loss of the foamed dielectric has
been offset by the advantage of the foam in blocking the
transmission of moisture into and through the cable, thereby
eliminating the need for gas pressurization or evacuation systems
to keep moisture out of the cable. Moisture, of course, greatly
increases the losses in coaxial cables and may, in fact, render
such cables inoperative.
Over the years since the introduction of foam dielectric cables, a
number of different dielectric compositions have been used and/or
proposed for use. Additionally, a number of different techniques
have either been used or proposed for use: (i) to foam the
dielectric resin; (ii) to apply the dielectric resin to the cable;
(iii) to control the size, uniformity and structure of the cells in
the foam; and (iv), to treat the foam after it has been formed. For
example, a number of different dielectric materials and blowing
agents or gas sources have been used or proposed for use in the
manufacture of such cables. The foaming of the dielectric plastic
resin has generally been effected either by the incorporation of a
chemical blowing agent in the molten resin which is then thermally
decomposed, or by the injection of a volatile fluid directly into
the molten resin during extrusion thereof. The direct injection
technique makes it more difficult to control the density and cell
size of the foam, but produces a lower loss foam without the
necessity of a drying step to remove moisture, which is one of the
reaction products produced by some of the chemical blowing agents.
The present assignee's Canadian Pat. No. 931,719 discloses a
process which used a combination of both of the foregoing foaming
techniques. Still another method involves swelling the resin in a
suitable solvent and, thereafter, extruding the swollen resin at a
temperature well above the boiling point of the solvent.
Certain of the foam jacketed and/or dielectric cables heretofore
made have had the foam adhesively bonded to the conductor to more
firmly "lock" the foam and conductor together and/or to insure
blockage of fluid flow along the interface between the foam and the
conductor. Bonding of the foam to the conductor has also been
effected by heating the conductor. Other foam dielectric coaxial
cables have been made without any bond between the foam and the
conductors, still achieving uniform spacing between the inner and
outer conductors and relatively tight engagement of the foam with
the inner conductor. It is believed that any and all of the
foregoing techniques may be used with compositions and processes
embodying the present invention.
The melt extrudable resin used in the present invention is a
perfluorocarbon resin, copolymers of tetrafluoroethylene and
hexafluoropropylene. Such perfluorocarbon materials are commonly
referred to as "fluorinated ethylene-propylene" (FEP) polymers. In
the aforesaid Randa U.S. Pat. No. 3,072,583 it has been recognized
that FEP polymers may be easily fabricated and possess excellent
properties in terms of dielectric strength and high melting point,
thereby making such materials particularly suitable for use as a
foamed dielectric in, e.g., coaxial cables or the like.
It has, therefore, been proposed in the aforesaid Randa U.S. Pat.
No. 3,072,583 that a melt extrudable FEP resin be foamed using a
fluoromethane--preferably, either dichlorodifluoromethane or
chlorodifluoromethane--blowing agent and boron nitride as a
nucleating agent. However, I have found that the use of boron
nitride as a nucleating agent imposes severe undesired constraints
on both the foaming process and the characteristics of the
resulting foamed product; apparently because if sufficient boron
nitride is added to the FEP resin to produce a small cell
structure--e.g., cells on the order of 1 to 40 mils. in
diameter--the melt strength of the resinous material is
significantly decreased, thereby precluding blowing the resin to
densities as low as, e.g., 0.5 g./cc. Rather, with small cellular
structure on the order of 1 to 40 mils., it appears that the
densities normally achievable are on the order of from 0.93 g./cc.
to 1.5 g./cc. Conversely, if the amount of boron nitride added to
the FEP resin is reduced so as to maintain relatively high
strengths for the resinous material, thereby permitting blowing of
the material to low density, the cell structure is degraded and
cell size becomes objectionably large. It is believed that the
foregoing problem--viz., an inability to obtain both (i) uniform
small sized cell structure and (ii) low density--is, at least to a
degree, further compounded when using fluromethane blowing
agents.
SUMMARY OF THE INVENTION
It is a general aim of the present invention to provide improved
foamable compositions including melt extrudable perfluorocarbon
resins and methods for preparing products incorporating such
compositions, where all of the foregoing disadvantages inherent in
the prior art are effectively overcome. In this connection, it is
an object of the invention to provide foamable compositions
incorporating a nucleating agent which is compatible with the
perfluorocarbon resin and which does not, at least to any
significant extent, decrease the melt strength of the resinous
material; and which, therefore, permits blowing the resinous
perfluorocarbon material to form a foamed cellular structure
characterized by both its relatively small uniform cell size and by
its relatively low density.
A principal object of the present invention is the provision of an
improved foamable perfluorocarbon composition and forming therewith
insulating jackets on electrical conductors as well as for forming
a foam dielectric material in a coaxial cable; and, wherein the
foamed composition is characterized by its stability at relatively
high temperatures, e.g., 200.degree. C., (392.degree. F.); its fire
resistance; its uniform small cell size and low density
(characteristics which are particularly desirable in coaxial cables
and which are also advantageous in insulating jackets for
electrical conductors); and its significantly improved electrical
performance in terms of reduced insulation loss and attenuation
(characteristics highly important in coaxial cables).
A related, but more specific object of the invention, is to provide
an improved method for preparing a cable in which the foamed
perfluorocarbon dielectric resin has a lower density than in
previously foamed perfluorocarbon dielectric compositions; yet,
wherein the foamed perfluorocarbon resin has the requisite
relatively small cell size, uniformity, structure, and composition
to provide the necessary structural strength and moisture blocking
and electrical characteristics.
A further object of the invention is to provide a process that is
suitable for manufacturing foamed dielectric cables and foamed
jacketed electrical conductors of the foregoing type on a
commercial scale by extruding the foamable composition directly
onto a conductor. Thus, a related object is to provide such a
process that can be carried out efficiently and economically.
In one of its more detailed aspects, it is an object of the
invention to provide a composition including a melt extrudable
perfluorocarbon resin and a process for foaming such resin which
permits forming such a foamed composition at considerably lower
temperature ranges than heretofore possible--e.g., at process
temperatures on the order of only 500 to 620.degree. F. as
contrasted with prior art processes which require temperatures in
excess of 680.degree. F.--thereby providing significant savings in
terms of energy consumption. A related object is to provide such a
composition and process which minimize corrosion of the processing
equipment.
DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the present invention
will become more readily apparent upon reading the following
detailed description and upon reference to the attached drawing, in
which:
FIG. 1 is a highly diagrammatic schematic illustration of a process
for producing a foamed composition suitable, for example, for use
as a jacket on electrical conductors or as a foamed dielectric
coaxial cable core or the like, all in accordance with the present
invention;
FIG. 2 is a side elevation, partially in section, of a jacketed
electrical conductor wherein the jacket comprises a foamed
composition embodying the present invention;
FIG. 3 is a sectional view taken substantially along the line 3--3
in FIG. 2;
FIG. 4 is a side elevation, partially in section, of a coaxial
cable having a foamed dielectric resin formed in accordance with
the present invention; and,
FIG. 5 is a sectional view taken substantially along the line 5--5
in FIG. 4.
While the invention is susceptible of various modifications and
alternative forms, specific embodiments thereof have been shown by
way of example in the drawings and will herein be described in
detail. It should be understood, however, that it is not intended
to limit the invention to the particular forms disclosed, but, on
the contrary, the intention is to cover all modifications,
equivalents and alternatives falling within the spirit and scope of
the invention as expressed in the appended claims.
DETAILED DESCRIPTION
It has been generally known heretofore that there is a correlation
between the amount of dielectric present in a given length of
cable--e.g., the density of the foam dielectric--and the insulation
loss and attenuation characteristics of the cable. In general, the
insulation loss and attenuation decrease as the amount of
dielectric decreases. However, it has also been known that the
lower density foams are better thermal insulators, which tends to
impede the dissipation of heat from the inner conductor of a
coaxial cable and, therefore, tends to reduce the power rating of
the cable. Moreover, there has been difficulty in achieving
successful manufacture of a foam FEP dielectric cable with
insulation loss and attenuation characteristics approaching those
of the air dielectric cables. The present invention is capable of
providing a foam perfluorocarbon dielectric cable with low
insulation loss and attenuation characteristics which approximate
those of air dielectric cables, while also providing fire
resistance along with most of the advantages of previous foam
dielectric cables.
Cables made in accordance with the present invention are formed by
extruding a melt extrudable perflourocarbon resin directly onto an
electrical conductor (which may be precoated with an adhesive
material) while foaming the resin by injecting a blowing
agent--e.g., a volatile fluid such as a lower fluorocarbon having
only one or two carbon atoms--directly into the resin, and bonding
the foamed resin to the electrical conductor to form a continuous
unitary layer of foam dielectric with a uniform thickness along the
entire length and around the entire circumference of the conductor.
Alternatively, although not illustrated in the drawing, those
skilled in the art will appreciate that the volatile fluid may be
dissolved in the solid resin prior to the heating and/or extrusion
process in the manner described in the aforesaid Randa U.S. Pat.
No. 3,072,583. The proportions of melt extrudable perfluorocarbon
resin and volatile fluid are controlled to form a closed cell foam
with a density at least as low as 1.0 g./cc. and an insulation loss
of less than 1.8 db/100 ft. at 1000 MHz.
A process for manufacturing a jacketed electrical conductor--which
may comprise the core of a coaxial cable--according to the present
invention has been schematically illustrated in FIG. 1. As there
shown, an electrical conductor 10 is withdrawn from a supply reel
12 and then chemically cleaned and passed through a heater 13,
after which the heated conductor is preferably coated with an
adhesive. When employed, such an adhesive coating extends
continuously along the entire length of the inner conductor and
around the entire circumference thereof. From the heater 13, the
electrical conductor 10 is passed through the crosshead of an
extruder 14 where a layer of foamable dielectric melt extrudable
resin from a hopper 15 is formed around the conductor. To foam the
melt extrudable resin, and in the exemplary system illustrated in
FIG. 1, a pressurized volatile fluid is fed into the extruder 14
through a line 16 and is mixed with the molten melt extrudable
resin so that the dielectric layer formed around the conductor 10
foams after it exits from the extruder.
Within the extruder, the melt extrudable resin is heated above its
melt temperature prior to injection of the volatile fluid to obtain
thorough dissolution or mixing of the fluid throughout the resin.
Between the point where the volatile fluid is injected into the
resin and the exit die, the temperature is reduced to the desired
melt temperature--i.e., the temperature at which the resin exits
from the extruder--which is generally in the range of about
500.degree. F. to about 620.degree. F. for perfluorocarbon resins.
The melt temperature should be just high enough to permit the resin
to be foamed. Although not shown in detail in the drawing, it has
been found that excellent results are obtained when using a
co-rotating twin screw extruder and a standard wire coating
crosshead equipped with conventional tubing type tip and die, and a
vacuum as contrasted with pressure type cooling. Thus, such a
system produces a smooth surface coating without voids along the
inner conductor 10. After the foaming of the dielectric is
completed, the foam is cooled so as to form a foam perfluorocarbon
jacket or layer 17 having dielectric properties which can serve
either as the insulating jacket for an electrical conductor 20
(FIGS. 2 and 3) or, alternatively, which can comprise the
dielectric core of a coaxial cable 21 (FIGS. 4 and 5) and which is
capable of supporting an outer conductor in precisely spaced
relationship to the inner conductor. Such cooling may be effected
by blowing air onto the foam or by means of a water spray or bath
(not shown). The resulting jacketed conductor 20 is drawn through
the process line by a capstan 18 and wound onto a take-up reel
19.
When the jacketed conductor 20 shown in FIGS. 2 and 3 is to form a
portion of coaxial dielectric cable 21 of the type shown in FIGS. 4
and 5, an outer conductor 22 is subsequently formed around the foam
dielectric 17, typically by rolling a flat strip of metal around
the foam, welding the longitudinal edges of the strip to form a
closed tube, and then corrugating the tube into the foam dielectric
to complete the formation of a corrugated outer conductor.
An example of a final coaxial cable is illustrated in FIGS. 4 and
5, with the preferred annular corrugations in the outer conductor
22. The low density foam dielectric 17 preferably has a small
average cell size--e.g., on the order of from about 10 mils. to
about 40 mils.--with a high degree of uniformity in the radial
profile of cell size distribution. As mentioned previously, the
foam dielectric in the cable provided by this invention exhibits a
density of at least as low as 1.0 g./cc. and an insulation loss of
less than about 1.8 db/100 ft. at 1000 MHz.
In accordance with one of the important aspects of the present
invention, provision is made for controlling the size of the cells
formed in the melt extrudable perfluorocarbon resin without the
addition of materials which tend to degrade the melt strength of
the resin, thereby permitting blowing of the resin to relatively
low densities--e.g., densities at least as low as 1.0 g./cc.--while
simultaneously obtaining a uniform cell structure in which the
individual cells formed are, on average, only 10 to 40 mils. in
size. In keeping with this aspect of the invention, I prefer to use
a nucleating agent for this purpose which is compatible with the
resin being used; and, I have found that particularly excellent
results are achieved where the nucleating agent comprises PTFE,
either in the pure state such, e.g., as in the form of "Hostaflon
TF 1620", a product commercially available from American Hoechst
Corp. located in Leominster, Mass.; or with suitable additives
such, e.g., as "DLX 6000", a product commercially available from E.
I. duPont De Nemours and Company, Wilmington, Del. The PTFE
nucleating agent is capable of providing the desired foam density
and cell size without significantly degrading the strength of the
perfluorocarbon resin; thus, the quantity of nucleating agent added
is really a function of the foamed resin's desired density and the
cell size desired. However, I have found that excellent results are
achieved where the PTFE nucleating agent has an average particle
size of less than 200 microns and is added to the resin in an
amount on the order of 0.5 to 2 parts PTFE to 100 parts of
resin.
When the compositions and products of the present invention are
used to form, for example, an insulated or jacketed electrical
conductor or a coaxial cable, the particular material from which
the conductors are formed is not critical to the invention and
virtually any suitable electrically conductive material can be
employed. However, in a high frequency coaxial cables the inner
conductor is usually formed of copper or aluminum and the same
metals are also generally used for the outer conductor. The inner
conductor is normally either a solid wire or rod of aluminum or
copper-clad aluminum, or a hollow copper tube with either a smooth
wall or a corrugated wall. The outer conductor may also be either
smooth walled or corrugated, although it is generally preferred to
use a corrugated outer conductor to improve the strength and
flexibility of the coaxial cable for any given metal thickness in
the outer conductor. This is particularly true in the case of
coaxial cables formed in accordance with the present invention
where the low density foam dielectric has less strength than a
higher density foam, so the higher strength offered by the
corrugated outer conductor is preferred in order to provide the
overall cable with the requisite strength. It is preferred that the
corrugations be of annular configuration, as opposed to helical, so
as to block the flow of moisture through the conductor-dielectric
interface without the necessity of adding a sealant.
When employing an adhesive to bond the foamed melt extrudable resin
to the conductor--for example, an adhesive such as shown at 11 in
FIGS. 2-5--it is desired that the adhesive coating on the inner
conductor adhere to both the metal of the inner conductor and the
dielectric material. Even more importantly, the adhesive must
provide sufficient bonding of the inner conductor to the dielectric
while the dielectric is at its melt temperature, to prevent the
foaming dielectric from sagging or blowing away from the inner
conductor after it exits from the extrusion die. The preferred
adhesive is a thin coating of solid perfluorocarbon resin, such as
"Teflon 100" available from du Pont. Because the adhesive
contributes to the insulation loss of the cable, it is preferred
that when an adhesive is to be used, it be used in the form of a
coating that is as thin as possible. In general, an adhesive
coating with a thickness in the range of about 0.002 inches to
about 0.004 inches is adequate to provide the requisite bonding in
the cables of this invention.
The volatile fluid or blowing agent that is introduced into the
extruder to foam the melt extrudable resin must be capable of
foaming the resin with a closed cell structure so that moisture
cannot be transmitted through the foam. It is preferred that the
average cell size in the final foamed dielectric be relatively
small, preferably in the range of from about 10 to about 40 mils.,
to provide optimum electrical and mechanical characteristics. As
mentioned previously, the gas that forms the cellular structure in
the dielectric must remain within the foam cells at least until the
foamed resin has solidified sufficiently to be
self-supporting--i.e., the gas must not permeate excessively
through the cell walls or membranes until the foamed resin has been
set. The volatile fluid must also be capable of being dissolved in
or thoroughly dispersed throughout the melt extrudable resin so as
to produce the desired uniform radial profile of cell size
distribution. Preferred blowing agents include the lower
fluorocarbons having only one or two carbon atoms such, for
example, as fluoroethanes represented by the formula: ##STR3##
where X is selected from the group consisting of fluorine,
chlorine, bromine and hydrogen. The preferred fluoroethane blowing
agent is 1,2-dichloro-1,1,2,2-tetrafluoroethane available from
duPont under the name "Freon 114".
While I have found that particularly advantageous results are
achieved when the blowing agent comprises a fluoroethane as
described above, the advantages of the present invention can also
be achieved, at least to a degree, when the blowing agent comprises
a fluoromethane represented by the formula: ##STR4## where X is
selected from the group consisting of fluorine, chlorine, bromine
and hydrogen--i.e., fluoromethanes such as those suggested in the
aforesaid Randa U.S. Pat. No. 3,072,583 such, for example, as
dichlorodifluoromethane and chlorodifluoromethane available from
duPont under the names "Freon 12" and "Freon 22" respectively.
Moreover, while I have herein described a composition and process
wherein the blowing agent is injected directly into the molten
extrudable resin, those skilled in the art will appreciate that the
blowing agent can be dissolved in the melt extrudable resin while
the latter is in solid form, either before or after the PTFE
nucleating agent is mixed therewith.
The following working example is given as an illustration of the
invention and is not intended to limit the scope of the
invention:
EXAMPLE
An inner conductor of copper clad aluminum wire having an outside
diameter of 0.185 inches and heated to a temperature of 520.degree.
F. was coated by extrusion with a film of "Teflon 100" having a
thickness of 0.003 inches. This coated inner conductor was then
passed axially through the center of the crosshead of a twin screw
extruder at a rate of 13 ft. per minute. The extruder was supplied
with 0.8 lbs. per minute of a mixture of "Teflon 100" and PTFE at a
ratio of 0.5 PTFE per 100 parts of "Teflon 100". Within the
extruder, the mixture of melt extrudable resin and PTFE was heated
to a maximum temperature of about 570.degree. F., and then cooled
to a melt temperature of 520.degree. F. at the die face. While the
melt extrudable resin and PTFE mixture was in the extruder, "Freon
114" was directly injected into the molten FEP-PTFE mixture at a
pressure of 700-800 p.s.i. at a point where the molten mixture was
at a temperature of about 560.degree. F. The extruder formed a
continuous layer of the FEP-PTFE-Freon mixture around the inner
conductor, and this layer began to expand as soon as it left the
extruder to form a layer of foam dielectric. This foam was air
cooled. The resulting cable core had a smooth uniform layer of foam
dielectric having a density of 0.78 g./cc., a thickness of 0.165
inches, a uniform cellular structure of non-interconnected cells 15
to 25 mils. in diameter, a void content of about 65% and a
dielectric constant of 1.38. The coating was smooth surfaced and
securely bonded to the inner conductor. An outer conductor of 0.010
inch copper was then formed around the core and corrugated to a
depth of 0.040 to 0.048 inches in an annular pattern having a
center-to-center corrugation spacing of 0.200 inch. The resulting
cable was tested for electrical performance with the following
results:
______________________________________ Velocity 85% Attenuation at
1000 MHz 3.45 db/100 ft. Impedance 50 ohms. Insulation loss at 1000
MHz 1.33 db/100 ft. Average power rating at 1000 MHz 1.29 KW. (at
300.degree. F. inner conductor temperature)
______________________________________
As can be seen from the foregoing detailed description, the present
invention provides improved compositions and processes for forming
foamed perfluorocarbon articles and, in particular, improved
compositions and processes for forming improved jacketed electrical
conductors and/or improved perfluorocarbon foam dielectric coaxial
cables. When used to form perfluorocarbon foam dielectric coaxial
cables, the invention provides thermal stability and fire
resistance combined with significantly improved electrical
performance--particularly, reduced insulation losses and
attentuation--as compared with previous perfluorocarbon dielectric
cables. Because the PTFE nucleating agent is compatible with the
melt extrudable resins utilized, it does not, to any significant
extent, degrade the melt strength of the resin and permits such
resins to be blown to lower densities and with a uniform cellular
structure having non-interconnected cells 10 to 40 mils. in
diameter. The foam thus formed has the requisite cell size,
uniformity and structure to provide the necessary structural
strength and moisture blocking characteristics for both jacketed
electrical conductors and coaxial dielectric cables. The improved
coaxial cables also tend to have improved power ratings.
Furthermore, jacketed electrical conductors and improved coaxial
dielectric cables can be efficiently and economically manufactured
on a commercial scale.
* * * * *